The gastropod shell is part of the body of a

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

or snail, a kind of

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. The shell is an

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some gastropods appear shell-less (

slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a smal ...

s) but may have a remnant within the mantle, or in some cases the shell is reduced such that the body cannot be retracted within it (

semi-slug Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but not quite vestigial. The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered ...

). Some snails also possess an operculum that seals the opening of the shell, known as the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

, which provides further protection. The study of

mollusc shell The mollusc (or molluskOften spelled mollusk shell in the USA; the spelling "mollusc" are preferred by ) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which ...

s is known as

conchology Conchology () is the study of mollusc shells. Conchology is one aspect of malacology, the study of molluscs; however, malacology is the study of molluscs as whole organisms, whereas conchology is confined to the study of their shells. It includ ...

. The biological study of gastropods, and other molluscs in general, is

malacology Malacology is the branch of invertebrate zoology that deals with the study of the Mollusca (mollusks or molluscs), the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams, ...

. Shell morphology terms vary by species group.

Shell layers

The gastropod shell has three major layers secreted by the mantle. The calcareous central layer, tracum, is typically made of calcium carbonate precipitated into an organic matrix known as

conchiolin Conchiolins (sometimes referred to as conchins) are complex proteins which are secreted by a mollusc's outer epithelium (the mantle). These proteins are part of a matrix of organic macromolecules, mainly proteins and polysaccharides, that ass ...

. The outermost layer is the periostracum which is resistant to abrasion and provides most shell coloration. The body of the snail contacts the innermost smooth layer that may be composed of mother-of-pearl or shell nacre, a dense horizontally packed form of conchiolin, which is layered upon the periostracum as the snail grows.

Morphology

Gastropod shell morphology is usually quite constant among individuals of a species. Controlling variables are: * The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the

abalone Abalone ( or ; via Spanish , from Rumsen ''aulón'') is a common name for any of a group of small to very large marine gastropod molluscs in the family (biology), family Haliotidae. Other common name In biology, a common name of a taxon o ...

, low rates give highly coiled forms such as ''

Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www. ...

'' or some of the

Planorbidae Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod molluscs. Unlike most molluscs, the blood of ram's horn snails contains iron-based hemoglobin instea ...

. * The shape of the generating curve, roughly equivalent to the shape of the aperture. It may be round, for instance in the

turban shell Turbinidae, the turban snails, are a family of small to large marine gastropod molluscs in the superfamily Trochoidea.Bouchet, P. (2014). Turbinidae Rafinesque, 1815. Accessed through: World Register of Marine Species at http://www.marinespecie ...

, elongate as in the

cone shell ''Conus'' is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at ...

or have an irregular shape with a siphonal canal extension, as in the ''

Murex ''Murex'' is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly called "murexes" or "rock snails".Houart, R.; Gofas, S. (2010). Murex Linnaeus, 1758 ...

''. * The rate of translation of the generating curve along the axis of coiling, controlling how high-spired the resulting shell becomes. This may range from zero, a flat planispiral shell, to nearly the diameter of the aperture. * Irregularities or "sculpturing" such as ribs, spines, knobs, and varices made by the snail regularly changing the shape of the generating curve during the course of growth, for instance in the many species of ''

''. * Ontologic growth changes as the animal reaches adulthood. Good examples are the flaring lip of the adult

conch Conch () is a common name of a number of different medium-to-large-sized sea snails. Conch shells typically have a high spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point at both ends). In North Am ...

and the inward-coiled lip of the

cowry Cowrie or cowry () is the common name for a group of small to large sea snails, ocean, marine Gastropoda, gastropod Mollusca, mollusks in the family Cypraeidae, the cowries. The term ''porcelain'' derives from the old Italian language, Italia ...

. Some of these factors can be modelled mathematically and programs exist to generate extremely realistic images. Early work by

David Raup David M. Raup (April 24, 1933 – July 9, 2015) was a University of Chicago paleontologist. Raup studied the fossil record and the diversity of life on Earth. Raup contributed to the knowledge of extinction events along with his colleague Jack Se ...

on the analog computer also revealed many possible combinations that were never adopted by any actual gastropod. Some shell shapes are found more often in certain environments, though there are many exceptions. Wave-washed high-energy environments, such as the rocky

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...

zone, are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculptured forms become more common in quiet water environments. The shell of burrowing forms, such as the

olive The olive, botanical name ''Olea europaea'', meaning 'European olive' in Latin, is a species of small tree or shrub in the family Oleaceae, found traditionally in the Mediterranean Basin. When in shrub form, it is known as ''Olea europaea'' ...

and ''

Terebra ''Terebra'' is a genus of small to large-sized predatory sea snails, marine gastropod molluscs in the subfamily Terebrinae of the family Terebridae, the auger snails. Species in this genus do not possess a radula. Species Species in the gen ...

'', are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand. On land, high-spired forms are often associated with vertical surfaces, whereas flat-shelled snails tend to live on the ground. A few gastropods, for instance the

Vermetidae The Vermetidae, the worm snails or worm shells, are a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Littorinimorpha. The shells of species in the family Vermetidae are extremely irregular, and do ...

, cement the shell to, and grow along, solid surfaces such as rocks, or other shells.

Chirality

Most gastropod shells are spirally coiled. The majority (over 90%) of gastropod species have dextral (right-handed) shells, but a small minority of species and genera are virtually always sinistral (left-handed), and a very few species (for example ''

Amphidromus perversus ''Amphidromus perversus'' is a species of air-breathing land snail, a Terrestrial animal, terrestrial pulmonate gastropod mollusk in the family Camaenidae. MolluscaBase eds. (2020). MolluscaBase. Amphidromus perversus (Linnaeus, 1758). Accessed t ...

'') show a mixture of dextral and sinistral individuals. There occur also aberrantly sinistral forms of dextral species and some of these are highly sought by shell collectors. Antiplanes vinosa 001

If a coiled gastropod shell is held with the spire pointing upwards and the aperture more or less facing the observer, a dextral shell will have the aperture on the right-hand side, and a sinistral shell will have the aperture on the left-hand side. This chirality of gastropods is sometimes overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be a rare or abnormal sinistral one. Sinistrality arose independently 19 times among marine gastropods since the start of the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

. This left-handedness seems to be more common in freshwater and land pulmonates. But still the dextral living species in gastropods seem to account for 99% of the total number. The chirality in gastropods appears in early cleavage (

spiral cleavage In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size ...

) and the gene

NODAL Nodal homolog is a secretory protein that in humans is encoded by the ''NODAL'' gene which is located on chromosome 10q22.1. It belongs to the transforming growth factor beta superfamily (TGF-β superfamily). Like many other members of this superf ...

is involved. A more recent study (2013) correlates the asymmetric coiling of the shell by the left-right asymmetric expression of the decapentaplegic gene in the mantle.

Mixed coiling populations

In a few cases, both left- and right-handed coiling are found in the same population. Sinistral mutants of normally dextral species and dextral mutants of normally sinistral species are rare but well documented occurrences among land snails in general. Populations or species with normally mixed coiling are much rarer, and, so far as is known, are confined, with one exception, to a few genera of arboreal tropical snails. Besides ''

Amphidromus ''Amphidromus'' is a genus of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Camaenidae. The shells of ''Amphidromus'' are relatively large, from to in maximum dimension, and particularly colorf ...

'', the Cuban '' Liguus vittatus'' (Swainson),

Haiti Haiti (; ht, Ayiti ; French: ), officially the Republic of Haiti (); ) and formerly known as Hayti, is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean Sea, east of Cuba and Jamaica, and ...

an '' Liguus virgineus'' (Linnaeus) (family

Orthalicidae Orthalicidae (orthalicid land snails) are a family of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks are classified in the subfamily Orthalicoidea of the order Stylommatophora. MolluscaBase eds. (2021). Mollusca ...

), some

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

an ''

Partulina ''Partulina'' is a genus of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Achatinellidae. Species Species within the genus ''Partulina'' include: * '' Partulina confusa'' *†'' Partulina crassa'' ...

'' and many Hawaiian ''

Achatinella ''Achatinella'' is a tropical genus of colorful land snails in the monotypic Achatinellidae subfamily ''Achatinellinae''. Species are arboreal pulmonate gastropod mollusks with some species called Oʻahu tree snails or kāhuli in the Hawaiian la ...

'' (family

Achatinellidae Achatinellidae is a family of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the superfamily Pupilloidea. Taxonomy It was previously the only family in the superfamily Achatinelloidea (according to the taxonomy ...

), as well as several species of Pacific islands '' Partula'' (family

Partulidae Partulidae is a family of air-breathing land snails, terrestrial gastropod mollusks in the superfamily Pupilloidea.MolluscaBase eds. (2021). MolluscaBase. Partulidae Pilsbry, 1900. Accessed through: World Register of Marine Species at: http:// ...

), are known to have mixed dextral-sinistral populations. A possible exception may concern some of the European clausiliids of the subfamily Alopiinae. They are obligatory

calciphile A calcicole, calciphyte or calciphile is a plant that thrives in lime rich soil. The word is derived from the Latin 'to dwell on chalk'. Under acidic conditions, aluminium becomes more soluble and phosphate less. As a consequence, calcicoles grown ...

s living in isolated colonies on limestone outcrops. Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together. Soos (1928, pp. 372–385) summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species. Others have stated that these populations were not distinct, and the question is far from settled. The Peruvian clausiliid, '' Nenia callistoglypta'' Pilsbry (1949, pp. 216–217), also has been described as being an amphidromine species. The genetics of reverse coiling in a rare dextral mutant of another clausiliid, ''

Alinda biplicata ''Alinda biplicata'',Kantor Yu I., Vinarski M. V., Schileyko A. A. & Sysoev A. V. (published online on March 2, 2010). "Catalogue of the continental mollusks of Russia and adjacent territories". http://www.ruthenica.com/documents/Continental_Russ ...

'' (Montagu), has been studied by Degner (1952). The mechanism is the same as in ''

Radix peregra ''Peregriana peregra'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod in the family Lymnaeidae, the pond snails. Distribution and habitat This small pond snail is found in Europe, Newfoundland and northern A ...

'' (Müller), with the direction of coiling determined by a simple

Mendelian Mendelian inheritance (also known as Mendelism) is a type of biology, biological Heredity, inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, an ...

recessive.

Standard ways of viewing a shell

File:Valvata sincera shell lateral.jpg, Apertural view of shell of ''

Valvata sincera ''Valvata sincera'', common name the mossy valvata is a species of small freshwater snail with a gill and an operculum, an aquatic gastropod mollusk in the family Valvatidae ''Valvatidae'', the valve snails, is a taxonomic family of ver ...

'' File:Valvata sincera shell 2.jpg, Abapertural view of shell of ''Valvata sincera'' File:Valvata sincera shell basal.jpg, Umbilical view of shell of ''Valvata sincera'' File:Calliostoma bairdii drawing.jpg, This dorsal view of the living animal '' Calliostoma bairdii'' also shows an apical view of its shell In photographs or illustrations, a gastropod shell can be shown oriented in a number of standard ways: * apertural view: this is the most common viewing angle. The shell is shown in its entirety, with its aperture facing the viewer, and the apex at the top. If the aperture is on the right side when viewed like this, then the shell-coiling is "right-handed" or dextral; if the aperture is on the left side when viewed like this, the shell has "left-handed" or sinistral shell-coiling. * abapertural view (or dorsal view): the shell is shown with its aperture 180° away from the viewer, and with the apex at the top. * umbilical view (or basal view): the shell is shown viewed directly from below. In most cases where there is an umbilicus, this is in clear view. * apical view: the shell is shown looking down directly onto the apex.

Description

The shell begins with the larval shell, the (usually) minute

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic whorls known as the

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called ...

, which is often quite distinct from the rest of the shell and has no growth lines. From the protoconch, which forms the apex of the

spire A spire is a tall, slender, pointed structure on top of a roof of a building or tower, especially at the summit of church steeples. A spire may have a square, circular, or polygonal plan, with a roughly conical or pyramidal shape. Spires are ...

, the coils or

whorls A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral ...

of the shell gradually increase in size. Normally the whorls are circular or elliptical in section. The spire can be high or low, broad or slender, according to the way the coils of the shell are arranged, and the apical angle of the shell varies accordingly. The whorls sometimes rest loosely upon one another (as in ''

Epitonium scalare ''Epitonium scalare'', common name the precious wentletrap, is a predatory or ectoparasitic species of marine gastropod with an operculum, in the family Epitoniidae, the wentletraps.Rosenberg, G. (2010). Epitonium scalare (Linnaeus, 1758). In: ...

''). They also can overlap the earlier whorls such that the earlier whorls may be largely or wholly covered by the later ones. When an angulation occurs, the space between it and the suture above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be smooth or keeled, and may sometimes have nodes or spines. The most simple form of

sculpture Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...

of the gastropod shell consists of longitudinal ridges, and/or transverse ridges. Primary spirals may appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals may appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types. Tertiary spirals are intercalated between the preceding groups in more specialized species. Ribs are regular transverse foldings of the shell, which generally extend from the suture to suture. They are usually spaced uniformly and crossed by the spirals. In specialized types, when a shoulder angle is formed, they become concentrated as nodes upon this angle, disappearing from the shoulder above and the body below. Spines may replace the nodes in later stages. They form as notches in the margin of the shell and are subsequently abandoned, often remaining open in front. Irregular spines may also arise on various parts of the surface of the shell (see ''

Platyceras ''Platyceras'' is a genus of extinct Paleozoic sea snails, marine gastropod mollusks in the family Platyceratidae. This genus is known from the Silurian to the Middle Triassic periods and especially abundant in the Devonian and Carbonifero ...

''). When a row of spines is formed at the edge or outer lip of the shell during a resting period, this feature sometimes remains behind as a

varix A varix (pl. varices) is an abnormally dilated vessel with a tortuous course. Varices usually occur in the venous system, but may also occur in arterial or lymphatic vessels. Examples of varices include: * Varicose veins, large tortuous veins usua ...

as in (''

'') and many of the

Ranellidae The Ranellidae, common name the triton shells or tritons, are a taxonomic family of small to very large predatory sea snails, marine gastropod mollusks in the order Littorinimorpha. Subfamilies According to the taxonomy of the Gastropoda by B ...

. Varices may also be formed by simple expansion of the outer lip, and a subsequent resumption of growth from the base of the expansion. The

or peristome of the shell may be simple or variously modified. An outer and an inner (columellar) lip are generally recognized. These may be continuous with each other, or may be divided by an anterior notch. This, in some types (''

Fusinus ''Fusinus'' is a genus of small to large sea snails, marine gastropod mollusks in the family Fasciolariidae, the spindle snails and tulip snails. MolluscaBase eds. (2020). MolluscaBase. Fusinus Rafinesque, 1815. Accessed through: World Reg ...

'', etc.) it is drawn out into an anterior

siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is ...

, of greater or lesser length. An upper or posterior notch is present in certain taxa, and this may result in the formation of a ridge or shelf next to the suture ('' Clavilithes''). An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the progressive closing of this slit may give rise to a definitely marked slit band. In some cases the slit is abandoned and left as a hole (

Fissurellidae Fissurellidae, common name the keyhole limpets and slit limpets, is a taxonomic family of limpet-like sea snails, marine gastropod molluscs in the clade Vetigastropoda.Rosenberg, G. (2012). Fissurellidae. Accessed through: World Register of Marin ...

), or by periodic renewal as a succession of holes (''

Haliotis ''Haliotis'', common name abalone, is the only genus in the family Haliotidae. This genus once contained six subgenera. These subgenera have become alternate representations of ''Haliotis''. The genus consists of small to very large, edible, h ...

''). The outer emargination is often only indicated by the reflected course of the lines of growth on the shell. On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like (''

Nerinea †''Nerinea'' is an extinct genus of fossil sea snails, marine gastropod molluscs in the clade Heterobranchia. Fossil record This genus is present from the Jurassic to the Cretaceous periods. Fossils are known from various localities of Europ ...

''). Similar ridges or columellar plicae or folds are more often found on the inner lip, next to the columella or central spiral twist. These may be oblique or normal to the axis of coiling (horizontal), few or numerous, readily seen, or far within the shell so as to be invisible except in broken shells. When the axis of coiling is hollow (perforate spire) the opening at the base constitutes the umbilicus. The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of the inner lip (''

Natica ''Natica'' is a genus of small to medium-sized predatory sea snails, marine gastropods in the subfamily Naticinae of the family Naticidae, the moon snails. The genus was erected by Giovanni Antonio Scopoli in 1777.Gofas, S. (2011). Natica Scop ...

''). Many Recent shells, when the animal is alive or the shell is freshly empty, have an uppermost shell layer of horny, smooth, or hairy epidermis or

periostracum The periostracum ( ) is a thin, organic coating (or "skin") that is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs, it is primarily seen in snails and clams, i.e. in gastropods and ...

, a proteinaceous layer which sometimes is thick enough to hide the color markings of the surface of the shell. The periostracum, as well as the coloration, is only rarely preserved in fossil shells. The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost. In life, when the soft parts of these snail are retracted, in some groups the aperture of the shell is closed by using a horny or calcareous operculum, a door-like structure which is secreted by, and attached to, the upper surface of the posterior part of the foot. The operculum is of very variable form in the different groups of snails that possess one.

Parts of the shell

The terminology used to describe the shells of gastropods includes: *

Aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

: the opening of the shell *

Lip The lips are the visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be ...

: ''peristome'': the margin of the aperture *

Apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics), a teenaged super villainess in the Marvel Universe * Ape-X, a super-intelligent ape in the Squadron Supreme universe *Apex ...

: the smallest few whorls of the shell *

Body whorl The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, the b ...

(or last whorl): the largest whorl in which the main part of the visceral mass of the mollusk is found *

Columella Lucius Junius Moderatus Columella (; Arabic: , 4 – ) was a prominent writer on agriculture in the Roman Empire. His ' in twelve volumes has been completely preserved and forms an important source on Roman agriculture, together with the wo ...

: the "little column" at the axis of revolution of the shell * Operculum: the "trapdoor" of the shell *

Parietal callus A parietal callus is a feature of the shell anatomy of some groups of snails, i.e. gastropods. It is a thickened calcareous deposit which may be present on the parietal wall of the aperture of the adult shell. The parietal wall is the margin of ...

: a ridge on the inner lip of the aperture in certain gastropods *

Periostracum The periostracum ( ) is a thin, organic coating (or "skin") that is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs, it is primarily seen in snails and clams, i.e. in gastropods and ...

: a thin layer of organic "skin" which forms the outer layer of the shell of many species *

Peristome Peristome (from the Greek ''peri'', meaning 'around' or 'about', and ''stoma'', 'mouth') is an anatomical feature that surrounds an opening to an organ or structure. Some plants, fungi, and shelled gastropods have peristomes. In mosses In mosses, ...

: the part of the shell that is right around the aperture, also known as the lip * Plait: folds on the columella. *

Protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called ...

: the nuclear or embryonic whorl; the larval shell, often remains in position even on an adult shell *

Sculpture Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...

: ornamentation on the outer surface of a shell **

Lira Lira is the name of several currency units. It is the current currency of Turkey and also the local name of the currencies of Lebanon and of Syria. It is also the name of several former currencies, including those of Italy, Malta and Israe ...

: raised lines or ridges on the surface of the shell *

Siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is ...

: an extension of the aperture in certain gastropods *

Spire A spire is a tall, slender, pointed structure on top of a roof of a building or tower, especially at the summit of church steeples. A spire may have a square, circular, or polygonal plan, with a roughly conical or pyramidal shape. Spires are ...

: the part of the shell above the body whorl. * Suture: The junction between

of most gastropods * Teleoconch : the entire shell without the protoconch; the postnuclear whorls. * Umbilicus: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus *

Varix A varix (pl. varices) is an abnormally dilated vessel with a tortuous course. Varices usually occur in the venous system, but may also occur in arterial or lymphatic vessels. Examples of varices include: * Varicose veins, large tortuous veins usua ...

: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices *

Whorl A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral ...

: each one of the complete rotations of the shell spiral

Shape of the shell

The overall shape of the shell varies. For example, three groups can be distinguished based on the height – width ratio:Falkner G., Obrdlík P., Castella E. & Speight M. C. D. (2001). ''Shelled Gastropoda of Western Europe''. München: Friedrich-Held-Gesellschaft, 267 pp. * oblong – the height is much bigger than the width * globose or conical shell – the height and the width of the shell are approximately the same * depressed – the width is much bigger than the height File:Bulgarica denticulata shell.png, oblong shell of '' Bulgarica denticulata'' File:Sphincterochila candidissima.jpg, globose shell of '' Sphincterochila candidissima'' File:Elona quimperiana shell 2.jpg, depressed shell of Escargot de Quimper The following are the principal modifications of form in the gastropod shell.

George Washington Tryon George Washington Tryon Jr. (20 May 1838 – 5 February 1888) was an American malacologist who worked at the Academy of Natural Sciences in Philadelphia. Biography George Washington Tryon was the son of Edward K. Tryon and Adeline Savidt. ...

, ''Structural and systematic conchology'', 1882, pp. 43–44 * Regularly spiral: ** Bulloid: bubble-shaped ''

Bulla Bulla (Latin, 'bubble') may refer to: Science and medicine * Bulla (dermatology), a bulla * Bulla, a focal lung pneumatosis, an air pocket in the lung * Auditory bulla, a hollow bony structure on the skull enclosing the ear * Ethmoid bulla, pa ...

'' ** Coeloconoid a slightly concave conical shell in which the incremental angle increases steadily during growth (see: ''

Calliostoma ''Calliostoma'' is a genus of small to medium-sized sea snails with gills and an operculum, marine gastropod molluscs within the family Calliostomatidae, the Calliostoma top snails. Previously this genus was placed within the family Trochidae ...

'') ** Cone-shaped, obconic. ''

Conus ''Conus'' is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at ...

'' ** Contabulate, short, with shouldered whorls ** Convolute: aperture as long as the shell, nearly or quite concealing the spire. ''

Cypraea ''Cypraea'' is a genus of medium-sized to large sea snails or cowries, marine gastropod mollusks in the family Cypraeidae, the cowries. Species Species within the genus ''Cypraea'' include: * † '' Cypraea ficoides'' (Hutton, 1873) * '' Cypra ...

'' ** Cylindrical, pupiform. '' Lioplax'', ''

Pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...

'' ** Cyrtoconoid: approaching a conical shape but with convex sides (see: ''

Gibbula ''Gibbula'' is a genus of small sea snails, marine gastropod molluscs in the subfamily Cantharidinae of the family Trochidae, the top snails.Bouchet, P.; Gofas, S. (2011). Gibbula Risso, 1826. Accessed through: World Register of Marine Spec ...

'') ** Depressed, lenticular. '' Ethalia carneolata'' ** Discoidal. ''

Elachorbis ''Elachorbis'' is a genus of minute sea snails or micromolluscs, Marine (ocean), marine gastropod molluscs in the family (biology), family Tornidae. Taxonomy The exact placement of this genus has long been contested. Tom Iredale establishe ...

'' ** Ear-shaped. ''

'' ** Elongated, subulate, elevated. ''

'' ** Few-whorled. ''

Helix pomatia ''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...

''. ** Fusiform, spindle-shaped. ''

'' ** Gibbous. Whorls swelled beyond the normal contour of increase (usually on the aperture side ). ''

Streptaxis ''Streptaxis'' is a genus of air-breathing land snails, terrestrial molluscs, terrestrial pulmonate gastropod mollusks in the family Streptaxidae. MolluscaBase eds. (2021). MolluscaBase. Streptaxis Gray, 1837. Accessed through: World Register of ...

''. ** Globular. ''

'' ** Many-whorled. '' Millerelix peregrina''. ** Short, bucciniform. ''

Buccinum ''Buccinum'' is a genus of medium-sized sea snails, marine gastropod molluscs in the family Buccinidae, the true whelks.Gofas, S.; Bouchet, P. (2014). Buccinum Linnaeus, 1758. Accessed through: World Register of Marine Species at http://www. ...

'' ** Trochiform, pyramidal, conical with a flat base. ''

Trochus ''Trochus'' is a genus of medium-sized to large, top-shaped sea snails with an operculum and a pearly inside to their shells, marine gastropod molluscs in the subfamily Trochinae of the family Trochidae, the top snails.Bouchet, P.; Gofas, S. (2 ...

'' ** Turbinated: conical, with rounded base. ''

Turbo In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

'' ** Turreted, turriculate, babylonic; an elongated shell with the whorls angulated or shouldered on their upper part. ''

'' ** Scalariform, whorls not impinging. ''

'' * Irregularly spiral, evolute. '' Siliquaria'', ''

Vermetus ''Vermetus'' is a genus of sea snails, marine gastropod mollusks in the family Vermetidae, the worm snails or worm shells. The genus ''Vermetus'' is very ancient: it occurs in the fossil record from the Jurassic to the Quaternary (age range: ...

'' * Tubular. * Shield-shaped. ''

Umbraculum The umbraculum ( it, ombrellone, "big umbrella", in basilicas also conopaeum) is a historic piece of the papal regalia and insignia, once used on a daily basis to provide shade for the pope (Galbreath, 27). Also known as the pavilion, in modern ...

'' * Boat-shaped, slipper-shaped. ''

Crepidula ''Crepidula'', commonly known as the slipper snails, slipper limpets, or slipper shells, is a genus of sea snails, marine (ocean), marine gastropod mollusks in the family Calyptraeidae. This family includes the slipper snails (''Crepidula''), hat ...

'' * Conical or limpet-shaped. ''

Patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

'' * Biconic: shaped like two conical shapes that are touching their bases, and tapering at both ends: ''

Fasciolaria tulipa ''Fasciolaria tulipa'', common name the true tulip, is a species of large sea snail, a marine gastropod mollusk in the family Fasciolariidae. Distribution This species occurs from the North Carolina coast all the way south and west to the Gul ...

'' * Pear-shaped: a combination of two shapes: ovate-conic and conic. ''

Ficus ''Ficus'' ( or ) is a genus of about 850 species of woody trees, shrubs, vines, epiphytes and hemiepiphytes in the family Moraceae. Collectively known as fig trees or figs, they are native throughout the tropics with a few species extending in ...

'' Detailed distinction of the shape can be: File:Cellana stellifera shell 2.jpg, cap shape File:Haliotis asinina.jpg, ear shape File:Theodoxus danubialis.jpg, neritiform File:Valvata cristata drawing.jpg, planispiral File:Valvata macrostoma.png, depressed trochiform or valvatiform File:Valvata piscinalis shell.jpg, trochiform File:Hydrobia ventrosa shell.jpg, ovate-conic File:Pyrgula annulata shell.jpg, conic File:VisTachetéI.png, elongate-conic or turriform or cockscrew shape File:Entemnotrochus rumphii.jpg, top shape File:Syrinx aruanus shell.jpg, spindle shape – the sea snail ''

Syrinx aruanus ''Syrinx aruanus'', common name the Australian trumpet or false trumpet, is a species of extremely large sea snail measuring up to 91 cm long and weighing up to 18 kg. It is a marine gastropod mollusk in the family Turbinellidae, and ...

'' has the largest shell of any living gastropod. File:Murex pecten shell 3.jpg, club shape -

Venus Comb Murex The Venus comb murex, scientific name ''Murex pecten'', is a species of large predatory sea snail, a marine gastropod mollusk in the family Muricidae, the rock snails or murex snails. The shell of this Indo-Pacific species has a very long siph ...

File:Bullata guerrinii.jpg, egg shape File:Lambis scorpius shell.jpg, irregular shape Gastropod shell measuring

Dimensions

The most frequently used measurements of a gastropod shell are: the height of the shell, the width of the shell, the height of the aperture and the width of the aperture. The number of whorls is also often used. In this context, the height (or the length) of a shell is its maximum measurement along the central axis. The width (or breadth, or diameter) is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal. The largest height of any shell is found in the marine snail species ''

'', which can be up to 91 cm.Wells F. E., Walker D. I. & Jones D. S. (eds.) (2003)
Food of giants – field observations on the diet of ''Syrinx aruanus'' (Linnaeus, 1758) (Turbinellidae) the largest living gastropod
. The Marine Flora and Fauna of Dampier, Western Australia. Western Australian Museum, Perth. The central axis is an imaginary axis along the length of a shell, around which, in a coiled shell, the whorls spiral. The central axis passes through the columella, the central pillar of the shell.

Evolutionary changes

Among proposed roles invoked for the variability of shells during

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

include mechanical stability, defense against predators and climatic selection.Goodfriend G. A. (1986) "Variation in land-snail shell form and size and its causes – a Review". ''Systematic Zoology'' 35: 204–223. The shells of some gastropods have been reduced or partly reduced during their

. This reduction can be seen in all

s, in semi-slugs and in various other marine and non-marine gastropods. Sometimes the reduction of the shell is associated with a predatory way of feeding. Some taxa lost the coiling of their shell during evolution. According to

Dollo's law Dollo's law of irreversibility (also known as Dollo's law and Dollo's principle), proposed in 1893 by Belgian paleontologist Louis Dollo states that, "an organism never returns exactly to a former state, even if it finds itself placed in condition ...

, it is not possible to regain the coiling of the shell after it is lost. Despite that, there are few genera in the family Calyptraeidae that changed their developmental timing (

heterochrony In evolutionary developmental biology, heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in the ...

) and gained back ( re-evolution) a coiled shell from the previous condition of an uncoiled

limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...

-like shell.Collin R. & Cipriani R. (22 December 2003) "Dollo's law and the re-evolution of shell coiling". ''

Proceedings of the Royal Society B ''Proceedings of the Royal Society'' is the main research journal of the Royal Society. The journal began in 1831 and was split into two series in 1905: * Series A: for papers in physical sciences and mathematics. * Series B: for papers in life s ...

'' 270(1533): 2551–2555. .

Taphonomic Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...
implications

In large enough quantities, gastropod shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize. For example, in the

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...

, fossil hadrosaur eggshell is rare. This is because the breakdown of

tannins Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'', f ...

from local

coniferous Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extant ...

vegetation would have caused the ancient waters to become acidic. Eggshell fragments are present in only two

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

sites, both of which are predominated by the preserved shells of invertebrate life, including gastropods. It was the slow dissolution of these shells releasing calcium carbonate into the water that raised the water's pH high enough to prevent the eggshell fragments from dissolving before they could be fossilized.Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206–218. In: Mesozoic Vertebrate Life – New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.

Variety of forms

File:Turritella communis fossiel.jpg, '' Turritella communis'', many-whorled shell of tower snail File:5-Turritele.jpg, X-ray image of ''

'' File:Cypraea nebrites.jpg, Shell of marine cowry snail – ''

Cypraea nebrites ''Naria nebrites'', common name the false margined cowry, is a species of sea snail, a cowry, a marine gastropod mollusk in the family Cypraeidae, the cowries. Subspecies and formae * ''Naria nebrites ceylonica'' (f) Schilder, F.A. & M. Schilde ...

'' Helix pomatia MHNT.ZOO.2002.0.33.jpg, ''

'' File:Naturalis Biodiversity Center - ZMA.MOLL.347109 - Epitonium scalare.jpg, ''

'' spiral shell File:3-porcelaine-cyprea.jpg, X-ray image of ''

'' File:2-Dolium geant.jpg,

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

image of the shell of '' Tonna galea'' File:4-Triton.jpg, ''

Charonia ''Charonia'' is a genus of very large sea snail, commonly known as Triton's trumpet or Triton snail. They are marine gastropod mollusks in the monotypic family Charoniidae. Etymology The common name "Triton's trumpet" is derived from the Gree ...

'' File:6-Venus.jpg, ''

Murex pecten The Venus comb murex, scientific name ''Murex pecten'', is a species of large predatory sea snail, a marine gastropod mollusk in the family Muricidae, the rock snails or murex snails. The shell of this Indo-Pacific species has a very long ...

'' File:Gastropod thin section PP.jpg, Thin section in plane-polarized light of microscopic gastropod shell, from

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

lagoonal sediment of Rice Bay,

San Salvador Island San Salvador Island (known as Watling's Island from the 1680s until 1925) is an island and district of The Bahamas. It is widely believed that during Christopher Columbus's first expedition to the New World, this island was the first land he s ...

, Bahamas. Scale bar 500 µm.

References

This article incorporates public domain text from references,John. B. Burch (1962)
"How to Know the eastern land snails; pictured-key for determining the land snails of the United States occurring east of the Rocky Mountain Divide"
WM. C. Brown Company Publishers, Dubuque, Iowa. 214 pages.

Laidlaw F. F. & Solem A. (1961). "The land snail genus ''Amphidromus'': a synoptic catalogue". ''Fieldiana Zoology'' 41(4): 505–720. Grabau A. W. & Shimer H. W. (1909) ''North American Index Fossils'
''Invertebrates. Volume I.''
A. G. Seiler & Company, New York. page
page 582
��584. and CC-BY-2.0 text from reference.Pfenninger M., Hrabáková M., Steinke D. & Dèpraz A. (4 November 2005) "Why do snails have hairs? A Bayesian inference of character evolution". ''BMC Evolutionary Biology'' 5: 59.

External links

* tp://ftp.fao.org/docrep/fao/009/y4160e/y4160e08.pdf Gastropods by J. H. Leal– Information on some gastropods of the tropical Western Atlantic, specifically the Caribbean Sea, with relevance to the fisheries in that region
Radiocarbon Dating of Gastropod Shells
* Nair K. K. & Muthe P. T. (18 November 1961

''Nature'' 192: 674–675. . * Antonio Ruiz Ruiz, Ángel Cárcaba Pozo, Ana I. Porras Crevillen & José R. Arrébola Burgo
''Caracoles Terrestres de Andalúcia. Guía y manual de identificación''
303 pp., .
from website
{{Authority control Gastropod anatomy Mollusc shells